The long-term goal of this research project is to understand the role of ion cotransport proteins in the regulated transport of sodium chloride across epithelia. Accomplishment of this long-term goal may be beneficial to the treatment of diseases where sodium chloride transport is altered or impaired, including cystic fibrosis and hypertension. The first specific aim is to test the hypothesis that intracellular chloride is the signal for activation of Na-K-Cl cotransport during the process of sodium chloride secretion and absorption. A tissue culture model to test this hypothesis will be developed by co-transfecting the cystic fibrosis transmembrane conductance regulator (CTFR) and the secretory or absorptive form of the Na-K -Cl cotransporter into tissue culture cells, and the time course of changes in intracellular chloride concentration and activation of Na-K-Cl cotransport will be measured. The second specific aim is to clone, express, and functionally characterize a candidate K-Cl cotransporter that has been identified by homology to previously cloned Na-K-Cl cotransporters. Cloning a K-Cl cotransport protein will be important because of the role of K-Cl cotransport in cell volume regulation and in sodium chloride absorption in the distal and proximal tubules of the mammalian kidney.